The present invention is in the field of broadband sources, and more particularly, in the field of rare earthdoped superfluorescent fiber sources in which an optical fiber is doped with a medium that can lase and in which the optical fiber is pumped with a pump optical signal to generate an output signal to be detected by a detector.
Optical fibers are being used for an increasing number of applications. One such application is an optical fiber rotation sensor comprising a loop of optical fiber into which two light signals are introduced and caused to counterpropagate around the optical loop. Such rotation sensors are described, for example, in U.S. Pat. No. 4,410,275; U.S. Pat. No. 4,456,377; U.S. Pat. No. 4,687,330; U.S. Pat. No. 4,634,282; and U.S. Patent No. 4,637,722. These patents are hereby incorporated by reference herein. For such rotation sensors and for other optical fiber applications, it is desirable to have a stable well-controlled light source.
For some applications, such as certain optical fiber rotation sensors, a high power broadband optical energy source having a short temporal coherence length and no longitudinal mode structure at longer wavelengths is desirable. It has been demonstrated that using a broadband optical energy source in an optical fiber rotation sensor, for example, reduces phase errors caused by the Kerr effect. A broadband optical signal can also be . advantageously used to reduce phase errors in the combined optical signal from the loop of the rotation sensor caused by coherent backscattering (i.e., Rayleigh backscattering) and by polarization cross-coupling in the loop. See, for example, U.S. Pat. No. 4,773,759; U.S. patent application Ser. No. 488,732, filed on Apr. 26, 1983; and U.S. Pat. application Ser. No. 909,741, filed on Sep. 19, 1986; all of which are assigned to the assignee of the present application. These patents and patent applications are hereby incorporated by reference herein. A theoretical analysis regarding the broadband source requirement for fiber gyroscopes can be found in W. K. Burns, et al., "Fiber-Optic Gyroscopes with Broad-Band Sources," Journal of Lightwave Technology, Volume LT-1, Number 1, pp. 98-105, March 1983. This article is hereby incorporated by reference herein. Optical fiber rotation sensors also require sources with highly stable mean wavelengths with little thermal drift. A rotation sensor source must also have the ability to couple high power into the rotation sensor without creating large noise components (high signal/noise ratio). Finally, an ideal rotation sensor source preferably operates in higher wavelength region of the optical spectrum of the source in order to reduce any sensitivity to radiation.
Such broadband optical sources include, for example, superluminescent light emitting diodes, and the like. An exemplary superluminescent diode has a relatively broad optical linewidth (e.g. approximately 15 nm) at the optical wavelengths in the range of 800 to 850 nm, for example. However, for a given power input, exemplary superluminescent diodes may not provide an adequate amount of optical energy when compared to a laser, for example. More importantly, superluminescent diodes cannot be easily coupled to certain optical devices such as gyroscopes as the light emitted by superluminescent diodes is highly divergent. In particular, the power produced by a superluminescent diode is difficult to efficiently couple into single-mode fibers. Furthermore, it is known that the temperature stability of the emission wavelength of a typical superluminescent diode is not satisfactory for numerous applications. The mean wavelength of superluminescent diodes varies about -300 ppm/.degree.C., which is inadequate for high sensitivity gyroscope applications that often require a mean wavelength stable to about 1 ppm.
More recently, U.S. Pat. No. 4,637,025 to Snitzer, et al., described a superradiant light source that includes an optical fiber having a core doped with a selected active laser material such as Neodymium.
U.S. patent application Ser. No. 281,088, filed on Dec. 7, 1988, discloses a superfluorescent broadband fiber source comprising a fiber doped with laser material coupling to a multiplexing coupler. This application is assigned to the assignee of the present application. Such a superfluorescent source has good output power and easily couples to an optical fiber rotation sensor. It does not have longitudinal cavity modes and shows good thermal stability. Its spectrum is much broader than a resonant laser source. The above patent and patent application are hereby incorporated by reference herein.
U.S. patent application Ser. No. 176,739, filed on April 1988, describes a broadband light source which uses an optical fiber doped with a lasing material such as Neodymium. This application is assigned to the assignee of the present application and is hereby incorporated by reference herein. The optical fiber is pumped with a pump optical signal having a pump wavelength selected to cause spontaneous emission of an optical signal at a second wavelength different from the pump wavelength. The wavelength of the pump optical signal is selected to be outside the pump variable tuning range of the Neodymium-doped optical fiber (i.e., the range of pump wavelengths which stimulate emitted wavelengths having an average wavelength with a generally one-to-one correspondence to the pump wavelength). Pumping with a pump signal outside the pump variable tuning ranges causes the emitted light to have a broad spectral envelope of longitudinal modes having emission wavelengths corresponding to substantially all the pump variable tuning range.
Neodymium-doped superfluorescent fiber sources have therefore alleviated the problems raised by either resonant cavity lasers or superluminescent diodes. Such sources can deliver milliwatts of power into a fiber optic gyroscope with broadband spectra and good thermal stability.
In recent years, Erbium-doped fibers have received increasing attention as possible sources and for amplification purposes in the low loss fiber communication window at 1500 nm. It is possible to obtain a high gain when the Erbium dopant is properly doped into the fiber, typically a silica fiber. The light emitted by Erbium-doped fibers easily couples into other fibers with similar mode sizes. An Erbium-doped fiber is also thermally relatively stable. Additionally, Erbium-doped fibers emit longer wavelength light than Neodymium-doped fibers, which makes them less sensitive to radiation induced loss mechanisms.
A theoretical analysis of amplified spontaneous emission can be found in an article by E. Desurvire et al, "Amplification of Spontaneous Emission in Erbium-Doped Single-Mode Fibers," Journal of Lightwave Technology, Volume 7, No. 5, pp. 835-845 (May 1989). The operation of an Erbium-doped silica fiber as a superfluorescent source at 1535 nm and pumped at 980 nm is also reported in an article by P. R. Morkel, "Erbium-Doped Fibre Superluminescent Source for the Fibre Gyroscope," Springer Proceedings in Physics, Volume 44, in Optical Fiber Sensors, Springer-Verlag Berlin, Heidelberg 1989. This article analyses the variation of superfluorescent output power with the pump power and the fiber length and observes the dependence of the spectrum of the superfluorescent emission on fibre length, pump power and fiber temperature. However, that article does not propose any method for minimizing the thermal variations of the output spectrum.
Although both Neodymium- and Erbium-doped fiber sources have a much better temperature stability than superluminescent diodes and resonant cavity lasers, there still is a need for a high power broadband light source using an optical fiber structure with very little thermal drift.
It is therefore an object of the present invention to substantially improve the thermal dependence of those thermal sources by a factor between 5 and 10, thereby reducing the thermal variation to only a few ppm/.degree.C. and even in optimal conditions determined by the method of the present invention, to less than 1 ppm/.degree.C.